Add gdk_event_get_touch_id()

Just a helper function to get the touch ID from touch events, it
returns FALSE in any other case.

docs/reference/gdk/gdk3-sections.txt

@@ -781,11 +781,13 @@ gdk_event_get_root_coords
 gdk_event_get_scroll_direction
 gdk_event_get_state
 gdk_event_get_time
+gdk_event_get_touch_id
 gdk_event_request_motions
 gdk_events_get_angle
 gdk_events_get_center
 gdk_events_get_distance
 gdk_event_triggers_context_menu
+gdk_event_get_touch_id
 
 <SUBSECTION>
 gdk_event_handler_set

gdk/gdk.symbols

@@ -168,6 +168,7 @@ gdk_event_get_scroll_direction
 gdk_event_get_source_device
 gdk_event_get_state
 gdk_event_get_time
+gdk_event_get_touch_id
 gdk_event_get_type
 gdk_event_handler_set
 gdk_event_mask_get_type

gdk/gdkevents.c

@@ -1722,6 +1722,45 @@ gdk_event_get_screen (const GdkEvent *event)
   return NULL;
 }
 
+/**
+ * gdk_event_get_touch_id:
+ * @event: a #GdkEvent
+ * @touch_id: return location of the touch ID of a touch event
+ *
+ * If @event if of type %GDK_TOUCH_MOTION, %GDK_TOUCH_PRESS or
+ * %GDK_TOUCH_RELEASE, fills in @touch_id and returns %TRUE,
+ * else it returns %FALSE.
+ *
+ * Returns: %TRUE if the touch ID can be extracted from @event.
+ **/
+gboolean
+gdk_event_get_touch_id (const GdkEvent *event,
+                        guint          *touch_id)
+{
+  if (!event)
+    return FALSE;
+
+  if (event->type == GDK_TOUCH_MOTION)
+    {
+      if (touch_id)
+        *touch_id = event->motion.touch_id;
+      return TRUE;
+    }
+  else if (event->type == GDK_TOUCH_PRESS ||
+           event->type == GDK_TOUCH_RELEASE)
+    {
+      if (touch_id)
+        *touch_id = event->button.touch_id;
+      return TRUE;
+    }
+  else
+    {
+      if (touch_id)
+        *touch_id = 0;
+      return FALSE;
+    }
+}
+
 /**
  * gdk_set_show_events:
  * @show_events:  %TRUE to output event debugging information.

gdk/gdkevents.h

@@ -1173,6 +1173,9 @@ void       gdk_event_set_screen         (GdkEvent        *event,
                                          GdkScreen       *screen);
 GdkScreen *gdk_event_get_screen         (const GdkEvent  *event);
 
+gboolean   gdk_event_get_touch_id       (const GdkEvent  *event,
+                                         guint           *touch_id);
+
 void	  gdk_set_show_events		(gboolean	 show_events);
 gboolean  gdk_get_show_events		(void);